The present invention relates to methods and products for improving the performance of magnesium containing metal air battery/fuel cells in one or more ways including: increasing anode utilization efficiency (suppressing hydrogen evolution), increasing energy density, increasing power density or increasing cell voltage.
It is well known in the prior art that certain battery electrodes, especially those used in metal-air batteries/fuel cells, suffer from undesirable hydrogen evolution during their xe2x80x9cdischargexe2x80x9d in which they generate electrical power or when they are stored, due to corrosion and/or moderate energy density i.e. watt-hours/liter output and/or low cell voltage. These electrodes include those containing magnesium and aluminum and/or zinc alone or in combination, as examples. The production of hydrogen is described by commercial fuel cell (battery) suppliers (e.g. www.greenvolt.com/fuel cells.htm). This producer portrays this as a safety issue. However it also represents a waste of metal fuel.
It is well documented that magnesium suffers from parasitic hydrogen evolution in inorganic electrolytes. For example Antonyraj (Antonyraj, A. and C. O. Augustin, 1998, xe2x80x9cAnomalous Behaviour of Magnesium Anodes in Different Electrolytes at High Concentrationsxe2x80x9d, Corrosion Reviews, 16(1-2): 127-138) states xe2x80x9cwhen magnesium metal comes in contact with aqueous electrolytes, self-dissolution of the metal and the evolution of hydrogen take place simultaneouslyxe2x80x9d (see pg 131). Song et al. (Song, G. et al., 1997, xe2x80x9cThe Electrochemical Corrosion of Pure Magnesium in 1N NaClxe2x80x9d, Corrosion Science, 39(5); 855-875) indicate that xe2x80x9cunder free corrosion conditions, magnesium corrosion can be considered to occur by the interaction of local anodes and cathodesxe2x80x9d (see pg 871). Song et al. suggest that magnesium can be converted to hydride by the following electrochemical reaction (see pg 858);
Mg+2H++2exe2x88x92=MgH2xe2x80x83xe2x80x83(1) 
MgH2+H2O=Mg2++2OHxe2x88x92+2H2xe2x80x83xe2x80x83(2) 
Proof of this suggested mechanism is given by Nazarov et al. (Nazarov, A. P. et al., 1989, xe2x80x9cFormation of MgH2 on Electrochemical Dissolution of Magnesium in Aqueous Electrolytes, Zashchita Metallov, 25(5): 760-765).
U.S. Pat. No. 5,024,904, issued to Curiel, describes the use of metal anodes, preferably made of magnesium, aluminum or magnesium-aluminum alloy, in combination with salt containing electrolytes and air cathodes for purposes of producing portable, direct current electrical power. Testing of the Curiel prototype by the current inventors has revealed the following major weakness: magnesium utilization efficiency as low as 30% due to parasitic hydrogen evolution.
U.S. Pat. No. 4,908,281, issued to O""Callaghan describes the undesirable production of hydrogen on aluminum electrodes in aluminum air cells (pg 1 lines 63+). xe2x80x9cAs with other batteries this hydrogen can easily reach explosive concentrations.xe2x80x9d (page 2 lines 10 to 12). One of the purposes of the O""Callaghan invention is to create a system designed to properly vent hydrogen to help prevent explosions. The electrolyte is designed to flow upwards and over a weir to discharge aluminum hydroxide product into an electrolyte reservoir. Air is used to dilute hydrogen below explosive limits. Tuck (Tuck, Clive D. S., Modern Battery Technology, 489-490) also describes parasitic, gaseous hydrogen evolution on aluminum contained in aqueous electrolytes.
Quraishi et al. (Quraishi, M. A. et al., 1999, xe2x80x9cDithiobiurets: A Novel Class of Acid Corrosion Inhibitors for Mild Steel, Journal of Applied Electrochemistry) have described the inhibition of corrosion/hydrogen evolution on steel, in strongly acidic environments using dithiobiurets with the following structure: 
where R and Rxe2x80x2 are aryl substituted functional groups such as phenyl, tolyl and so on.
U.S. Pat. No. 5,004,654 issued to Hunter et al. describes the benefits of a source of tin e.g. tin containing ions such as stannate ions, on undesirable hydrogen evolution in aluminum/air cells.
U.S. Pat. No. 3,594,235 issued to Moran describes the use of quaternary ammonium salt containing electrolyte in combination with metal/air batteries (fuel cells) containing cadmium or magnesium electrodes. The use of quaternary ammonium salt as the sole electrolyte component other than water, especially at an excessively high concentration of 10% by weight, makes the Moran invention prohibitively expensive for non-military applications.
The prior art related to batteries, especially metal/air batteries (fuel cells) such as those including magnesium and aluminum and/or zinc, alone or in combination e.g. as alloys, has failed to incorporate knowledge in the use of hydrogen evolution inhibitors derived for steel, especially in highly acidic environments. Attempts to minimize deleterious evolution of hydrogen have been generally restricted to the use of exotic and/or expensive metal alloys.
Finally, the prior art related to magnesium/air batteries and fuel cells has failed to incorporate knowledge derived by the aluminum industry related to corrosion inhibition by tin containing electrolytes.
Accordingly, it is an object of the current invention to provide improved methods for inhibition of hydrogen evolution (improved anode utilization efficiency) and/or energy density and/or cell voltage and/or power density improvement in batteries, especially metal/air batteries (fuel cells), especially those containing magnesium, magnesium and aluminum, magnesium and zinc.
The invention relates to a method of improving the performance of magnesium containing electrodes used in metal/air batteries (fuel cells), comprising the addition of one or more additives to the electrolyte or electrode surface. More specifically it relates to performance improvement due to any one of the following factors alone or in combination: the inhibition of hydrogen evolution (improvement of electrode utilization), improvement of energy density, improvement of power density and/or increase in cell voltage. The additives are selected from any of the following groups; dithiobiuret, tin, and tin plus a quaternary ammonium salt.
Advantageously, dithiobiuret additives may be used, which have the following structure: 
in which either or both of the R and or Rxe2x80x2 function groups contain an aryl group (aromatic ring structure), for example, in which R is a tolyl group xe2x80x94C6H5xe2x80x94CH3 and Rxe2x80x2 is a phenyl group C6H5xe2x80x94.
Tin containing additives may be used either in the electrolyte or on the electrode surface, for example, in the form of stannate salts such as sodium stannate.
Tin containing additives may also be used either in the electrolyte or on the electrode surface, for example, in the form of stannate salts such as sodium stannate, in combination with a quaternary ammonium salt such as tricaprylmethylammonium chloride (e.g. Aliquate 336).
The invention also includes improved metal/air fuel cells and batteries based on the above methods.
The following non-limiting examples show the flexibility of the invention as applied to magnesium/air battery/fuel cells: